The invention relates generally to semiconductor device fabrication and, in particular, to methods of fabricating device structures, such as bipolar junction transistors and diodes, using a trench.
Bipolar junction transistors are typically found in demanding types of analog circuits, especially analog circuits used in high-frequency applications. For example, bipolar junction transistors are commonly used in radiofrequency integrated circuits (RFICs) found in wireless communications systems. Bipolar junction transistors may be combined with complementary metal-oxide-semiconductor (CMOS) field effect transistors in bipolar complementary metal-oxide-semiconductor (BiCMOS) integrated circuits, which take advantage of the favorable characteristics of both transistor types.
A typical bipolar junction transistor includes three semiconductor regions, namely the emitter, base, and collector regions, of which the emitter and collector regions have an opposite conductivity type in comparison with the base region. Generally, a bipolar junction transistor includes a pair of p-n junctions, namely an emitter-base junction and a collector-base junction. An NPN-type bipolar junction transistor consists of a thin region of p-type semiconductor material constituting the base region sandwiched between two regions of n-type semiconductor material constituting the emitter and collector regions. A PNP-type bipolar junction transistor has two regions of p-type semiconductor material constituting the emitter and collector regions and a thin region of n-type semiconductor material constituting the base region sandwiched between them.
A voltage applied across the emitter-base junction controls the movement of charge carriers that produce charge flow between the collector and emitter regions. Because the collector region surrounds the emitter region, almost all of the charge carriers injected from the emitter into the base region are collected. As a result, a small current entering the base region of an NPN-type bipolar junction transistor, or flowing from the base region in the case of a PNP-type bipolar junction transistor, is highly amplified in the collector output. Because electron mobility is higher than hole mobility in silicon, NPN-type bipolar junction transistors offer greater currents and faster operation under most circumstances than PNP-type bipolar junction transistors. As a consequence, NPN-type bipolar junction transistors are the favored type of bipolar junction transistor found in RFIC's or BiCMOS integrated circuits.
Conventional planar bipolar junction transistors are commonly fabricated with vertical stacked arrangement of the emitter, base, and collector regions. In silicon-on-insulator (SOI) technology, a top, thin layer of silicon or SOI layer is employed to build active devices. Fabrication of bipolar junction transistors having a conventional vertically stacked arrangement can be difficult in such thin layers of silicon. Similar considerations apply for diodes that include a single p-n junction, as well as other device structures that may contain more than two p-n junctions. Although lateral p-n junctions can be built in thin SOI layers, the ideality of such diodes suffers, and is typically greater than unity. This degradation in ideality makes such diodes less than ideal for many analog applications, such local temperature monitors or band-gap reference voltage generators.
What is needed, therefore, are methods for fabricating device structures, such as bipolar junction transistors and diodes, that overcome these and other disadvantages of conventional fabrication methods for these types of device structures.